Genes involved in the determination of segmental identity in Drosophila melanogaster have been identified on the basis of genetic interactions with homeotic mutations, mutations already known to affect the process. Mutations in previously-identified homeotic genes have been recovered, as well as mutations in new homeotic genes. One of the known homeotic genes, the trithorax locus, and four of the new homeotic genes (the genes kismet, brahma, moira, and osa) have been characterized in more detail. The kismet gene has been shown to encode a post-transcriptional regulator, coactivator, or function as a downstream target for the homeotic gene Antennapedia. The remaining four genes are transcriptional regulators of Antennapedia. The kismet gene has been cloned and spans >60 kb of genomic DNA. Two partially-overlapping transcripts (8 and 17 kb in length) appear to be derived from alternative promoters. The brahma gene has also been cloned and encodes a single 5.5 kb transcript present at all developmental stages. DNA sequence analysis of cDNA clones indicates that the brahma protein may include a conserved region found in three other proteins, the Drosophila fsh protein, the yeast spt7 protein, and the human CCG1 protein. The Drosophila brahma and fsh genes both exhibit similar genetic interactions with two other homeotic genes, suggesting a functional role for the conserved protein domain. The transcription unit adjacent to the brahma gene has been shown to encode a member of the superfamily of small GTP-binding proteins. As the predicted protein sequence is most similar to the yeast and mammalian ADP-ribosylation factor proteins (ARFs), the gene has been named arf-related (arl). The purified Drosophila protein binds guanine nucleotides and hydrolyzes GTP, but is unable to substitute for ARFs in an in vitro ARF functional assay. An embryonic-lethal arl mutation was isolated. arl is the first member of the superfamily of small GTP-binding proteins demonstrated to be essential in a multicellular eukaryote.